I don't know why, but I can provide some guesses:
1. any contact with a loop, or possibly just aerodynamic effects, risks messing up the engines/thruster control calculations and/or knocking the rocket over
2. loop clearance is going to have to be small if it is going to help (eg. if landing leg fails) but large to avoid (1) - maybe there is no right size
3. need to add in the effects of wave motion on loop and whatever structure is holding it up - for a start, the higher up it is the more it will move, relative to descent path, with the waves
4. a strong enough loop suspension structure may add significant weight to the barge, but more importantly moves it's CofG upwards, making it less stable and giving more roll in the waves, quite possibly negating any benefit from the loop (in terms of chance success)
But I think the big one is this: the rocket _looks_ hugely unstable on landing, and the little legs don't look wide enough, but this is deceptive. With most of the fuel gone and a lot of weight in the engines I bet the CofG of the rocket is probably much much lower than it intuitively looks. Think a long cardboard tube with a lead weight at the bottom - how much do you actually need to stabilise it to get it to stand up?
Now, wind might be a problem, but then it's at sea, if you have high winds you have big waves and you are stuffed anyway.
Last point: they only need to re-use some of the rockets to make launches a _lot_ cheaper, and they don't have much storage space left that they could have put this one in anyway :-)